This invention relates to adsorbent blends for the generation of oxygen from a gaseous mixture, wherein a particularly useful blend includes a zeolite X blended with a binder, wherein the binder includes highly dispersed attapulgite fibers and wherein the zeolite X is lithium exchanged, preferably after blending, and processes for the use of the adsorbent blend. This invention also relates to processes of production of adsorbent blends prepared by the blending of zeolite X with a binder to form a blend, which binder includes highly dispersed attapulgite fibers, and then ion exchanging the zeolite X component of the blend with lithium cations.
Zeolites are hydrated metal alumino silicates having the general formulaM2/nO:Al2O3:xSiO2:yH2Owhere M usually represents a metal, n is the valence of the metal M, x varies from 2 to infinity, depending on the zeolite structure type, and y designates the hydrated status of the zeolite. Most zeolites are three-dimensional crystals with a crystal size in the range of 0.1 to 30 μm. Heating these zeolites to high temperatures results in the loss of the water of hydration, leaving a crystalline structure with channels of molecular dimensions, offering a high surface area for adsorption of inorganic or organic molecules. The adsorption of these molecules is limited by the size of the zeolite channels. The rate of adsorption is limited by the laws of diffusion.
One limitation on the utilization of zeolite crystals is their extremely fine particle size. Large, naturally-formed agglomerates of these crystals break apart easily. Because the pressure drop through a bed containing zeolite particle is often prohibitively high, zeolite crystals alone cannot be used in fixed beds for various dynamic applications, such as the drying of natural gas, drying of air, separation of impurities from a gas stream, separation of liquid product streams, generation of oxygen from a gaseous mixture, and the like. Therefore, it is desirable to blend these crystals with binder materials to provide an agglomerate mass of the crystals, which exhibits a reduced pressure drop.
To permit the utilization of these zeolite crystals, different types of clays are conventionally used as binders with the crystals, including attapulgite, palygorskite, kaolin, sepiolite, bentonite, montmorillonite and mixtures thereof. The clay content of a blended zeolite can vary from as low as 1 percent to as high as 40 percent, by weight, although the preferred range is from about 10 to about 25 percent, by weight.
An adsorbent for separating gases comprising a binder and a crystalline, low silica faujasite-type zeolite with a silica to alumina molar ratio of 1.9 to 2.1 is disclosed in EP 0 940 174 A2. This reference discloses the blending of a zeolite with a conventional, dense attapulgite clay binder. The bulk density of the binder is not disclosed.
The use of a zeolite X having its base metal ions ion exchanged with lithium ions has also been disclosed. Such zeolites have been useful for the separation of nitrogen from oxygen. A process for the use of lithium exchanged zeolite X for nitrogen absorption is disclosed in U.S. Pat. No. 4,859,217.
One problem with zeolites blended with conventional binders is decreased diffusion. The larger the diameter of the zeolites, the slower the rate of diffusion of the molecules to be adsorbed. Particularly in the field of pressure swing adsorption, this effect is highly adverse to short cycle time and thus to productivity. Enhanced kinetic values or faster mass transfer rates can result in lower power consumption and higher adsorbent productivity.
It has been recognized that a reduction in the particle size of formed zeolites leads to shorter mass transfer zones and shorter cycle times. This is based on the assumption that the time needed for adsorbates to travel through the macropores of the adsorbents limits the cycle time, i.e. macropore diffusion is the rate limiting step in these processes. The problem can be partially solved by adding pore forming compounds to the zeolite/binder blend before the forming step.
Accordingly it is an object of the invention to disclose a process for the production of an adsorbent blend, which is especially useful for the generation of oxygen from a gaseous stream, a process for the use of that blend, and the composition of the blend.
These and other objects are obtained by the processes for production, the processes of use and products of the invention disclosed herein.